Tunable cavity antenna



Nov. 14, 1961 R. w. MASTERS ETAL 3,009,153 TUNABLE CAVITY ANTENNA Filed July 20, 1960 INVENTORS R. W. MASTERS 0o LDR.MCCOY ll g T AT ORNE Km 0.

AGEN

United States Patent 3,009,153 TUNABLE CAVITY ANTENNA Robert W. Masters, Columbus, Ohio, and Donald R. McCoy, Liverpool, N.Y., assignors to the United States of America as represented by the Secretary of the Air Force Filed July 20, 1960, Ser. No. 44,236 11 Claims. (Cl. 343-746) The invention is directed to a high efiicient miniature antenna which tunes linearly over approximately a one hundred percent frequency range and radiates nearly circularly polarized waves in a wide, angle beam pattern normally to its aperture. The aperture size varies upward from A; wave length in the practical range of operation. Tuning is accomplished by a continuously adjustable control which positions a modified piston type capacitor element inside the cavity by means of a lead screw.

An optimum solution for the problem of designing an efficient miniature cavity antenna for use over wide frequency ranges has long been sought. Because lossless dielectric and perfectly conducting metals are not available, the bandwidths of physical antennas must inevitably depend in a complex way upon the constants of the materials used in construction as well as upon the electrical size and geometry. For this reason, an empirical formula for the maximum bandwidth of a miniature cavity antenna in terms of aperture size and efiiciency has eluded the most determined experimental efforts. The bandwidths of antennas carefully constructed of good materials are seriously limited by a function which decreases rapidly with aperture size, regardless of geometry.

Further evidence in support of this lies in the fact that the Q of cavity antenna with minor restrictions may be interpreted as an inverse measure of bandwidth. It has been found that the miminum Q obtainable by the lossless dielectric filling of a rectangular cavity having a square aperture of length a is very nearly for apertures less than about four-tenths of a wavelength square.

Many workers in the art have noted the stringent restriction of the bandwidth as an overall antenna dimension becomes small in wavelengths. All evidence is to the effect that the realization of an efiicient, fixtuned, broadband (in the order of 50 to 100 percent), miniature cavity antenna is practically impossible.

To circumvent the apparently inescapable bandwidth 7 useful.

It is thus an object of the invention to provide an efficient miniature cavity antenna for use over wide frequency ranges.

It is another object of this invention to provide a highly efiicient miniature antenna which tunes linearly over approximately a one-hundred percent frequency range and radiates nearly circular polarized waves in a wide angle beam pattern normal to its aperture.

It is a further object of the invention to provide a tunable cavity antenna which has closely approached the optimum design, as regards intrinsic bandwidth, for efficient transmission through apertures of the order of 0.2 wavelength square.

In the drawings:

FIG. 1 is a front sectional view of the cavity antenna taken along line 11 of FIG. 2;

FIG. 2 is a side sectional view of the cavity antenna taken along line 2-2 of FIG. 1;

FIG. 3 is a schematic equivalent circuit of the tunable cavity antenna;

FIG. 4 is a schematic connection diagram for the power equalizer to the antenna;

FIG. 5 is a front sectional view taken along line 5-5 of FIG. 6 of a modified piston type capacitor; and

FIG. 6 is a side sectional view taken along line 66 of FIG. 5 of a modified piston type capacitor.

Referring now more particularly to FIGS. 1 and 2, an electrical small tunable cavity antenna which has a square aperture and operates in a manner quite similar to that of a symmetrical coaxial resonator is shown. The antenna comprises a boxlike metal housing 1, having continuous side walls 2 of equal length, and a back wall 3. The front side of the housing is the aperture of the antenna. In the aperture there is positioned a first pair of posts or radiators 5. The posts are attached to opposite sides of the housing in a facing relation and are spaced apart. A second set of posts 6 are preferably installed in the aperture at right angles to the first pair. The two pairs of posts or orthogonal radiators are so shaped that a symmetrical-cross opening is formed between the pairs of posts in the center of the aperture. Each of the posts has a passage 7 extending through its central portion. The passages for one pair of posts are at one level and the passages for the other pair of posts are at another level, so that the coaxial cable or line means 8 and 9, which extend through the passages of the first and second pairs of posts 5 and 6, respectively, may pass each other in the opening between the posts without contacting. At the end of each cable means is a short probe exciter 10 which is the portion of each cable means which extends across the opening between the pairs of posts and acts to excite the aperture. The piston type tuning plate 11, inside the housing 1, tunes both pairs of posts simultaneously. A lead screw 12 located outside of the housing adjusts the position of the tuning plate 11 to accomplish the proper tuning. A continuously adjustable control means to continuously position the piston type tuning plate 11 may be used to actuate the lead screw 12.

The criticalness of tuning and intrinsic bandwidth are the two limiting factors determining the lowest practical operating frequency for a given aperture. Bandwidths for aperture smaller than about A; wavelength square are exceedingly narrow. The construction of the tuner determines the upper operating frequency. A round figure for the practical operating bandwidth is one-hundred percent. That is, the aperture may be operated between A; and wavelength square.

The conductance of rectangular apertures less than about 0.4 wavelength square is very nearly proportional to the square of the frequency. That is,

The susceptance can be closely identified with that of an elementary parallel L-C network. The posts in the aperture have essentially the same effect as an iris to modify principally the aperture susceptance. Resonance'at'the aperture can, in fact, be induced by the proper shapeof the iris.

Since the antenna geometry is electrically small, one is led intuitively to represent its impedance function Z at the probe base by the equivalent circuit of FIG. 3. 'The conductance is assumed to have the same functional form as the aperture conductance G, the aperture susceptanceis lumped with that of the cavity in the L-C couplet, and

3 the variable capacitance of the tuning plate is shown as C The series reactance of the probe is denoted by X. Then,

which is a real constant, R, at any resonance if,

T X IH/ 1 (4) Since the aperture conductance decreases rapidly from about 0.001 mho as the aperture size shrinks from 0.4 wavelength square, one has l/RG1, whence, in view of Equation 2,

Thus, to obtain a constant imput resistance at resonance, X must vary inversely with the frequency. Since the sign of X is optional in virtue of the tuning control on B, a realizable element would consist of a series capacitance. This explains the use of the short probe 10. The focus of impedances at the probe input to the radiators is essentially independent of the resonant frequency.

The complete scheme of the tunable cavity antenna for circularly polarized radiation calls for the time and space quadrature excitation of the aperture. The means for this is supplied by the second pair of posts or radiators 6 installed in the aperture at right angles to the first pair in such a way as to avoid coupling between the two sets. Cables 8 and 9 differing in electrical lengths by nominally 1r/2 radians from a common junction to excite the two orthogonal radiators 5 and 6, which must be geometrically identical and entirely symmetrical in their arrangements. The tuning plate 11 thus tunes both elements simultaneously, and the one control is sutficient for wide range constant resistant tuning. The radiation pattern of the aperture is a broad beam normal to the aperture and having no nulls. It is circularly polarized on the beam axis and nearly so in regions near the beam axis. The power pattern is a figure of revolution about the beam axis.

A wideband power equalizer 15 is preferably inserted where circularly polarized radiation is desired in the main input line before the input to the antenna to produce stability of the input impedance and polarization characteristics for operation in a broad range of frequencies centered on the design frequency at which the polarization is perfectly circular. The power equalizer as illustrated in FIG. 4 comprises a ladder type resistor 16 and an input connection 17. The output of the power equalizer is applied through branch cables 18 and 19 to the inputs of the coaxial lines 8 and 9, respectively of the cavity antenna. A typical power equalizer-antenna arrangement for circularly polarized operation between 300 and 700 mc. would require a wavelength ladder type resistor and standard 50 ohm cables of length l and 1 plus M4 wavelength.

Excellent performance is realizable using a power equalizer because the waves reflected back into the feed system by impedance mismatches between the radiating elements and their branch lines are trapped out and largely absorbed by a damping resistor instead of being allowed to return to the input terminals.

It is further considered operationally advantageous to use the power equalizer whenever circularly polarized radiation is desired because of the narrow percentage bandwidth obtainable from the individual miniature cavity antenna elements. A nearly constant resistance can thus be presented to a transmitter or receiver, and the tuning of such auxiliary apparatus is greatly simplified.

Energy transferred between the crossed radiators by electromagnetic coupling appears as a reflection at the main input terminals of a circularly polarized antenna instead of being radiated or absorbed in the power equalizer. It is important, therefore, to minimize this coupling as much as possible, and the task is made delicate by the relatively high average stored energy in the aperture region. It is assumed already that the radiators are identical because this is necessary to insure equality of their impedance functions over a frequency range. A further obligation is that the geometry, especially in the aperture, be exceedingly symmetrical lest the so-called neutral planes be warped locally and coupling result. The aperture should, in fact, have perfect mirror symmetry about the normal planes containing axes of the radiators, and the axis should be orthogonal. The degree to which these conditions can be realized determines the success of the design with respect to coupling and tracking and tuning.

The sensitivity of coupling to slight deviations from perfect symmetry is incredible for these electrically small antennas, and it increases rapidly as electrical size decreases. Minute asymmetrical misalignment of the posts 5 and 6 in their spacing from the tuning capacitor 11 can cause noticeable coupling or failure to track at the low frequency end of the tuning range where the capacitor spacing is already small. The basic requirement of exquisite symmetry is far from satisfied, for instance, by unbalanced probes openly crossed near to each other in the gap. Coupling takes place through the small mutual capacitance and the main input impedance characteristic is adversely affected.

The cross coupling is virtually eliminated by use of shields in the form of cylindrical extensions 14 from the tips of the posts. The probe 10 can thereby be shielded to a small gap at the center of the aperture. They can also be offset at different enough levels in the cavity to permit crossing them. The gap width is not particularly critical, but, as might be expected, the probe length is. This solution is especially satisfactory because it does not compromise the impedance functions of the radiators in any way and the transition from a single coaxial cable to well-balanced excitation is strikingly simple and positive.

The offsetting of the probe shields to permit crossing the feeds is an unavoidable deviation from the ideal of perfect geometrical identity of the radiators but the electrical effect is quite small and easily compensated. The effect of the offsetting of the probe shields 14 is a slight difference in capacitance between the tuning plate 11 and the pairs of posts. This difference can be neutralized at the low end of the frequency range by increasing slightly the capacitance of the pair which is lacking. This is done by shimming the posts.

It is possible to achieve approximate linearity of tuning from 250 to 475 mc., by shaping the geometry of the piston type capacitor properly. FIGS. 5 and 6 illustrate such a means for accomplishing this result. Flange cylindrical extensions 20 are secured one to each of the posts 5 and 6 and the side walls of the housing 1. The modified tuning plate 22 has a cylindrical extension 23 extending below a cylindrical plate 24. The capacity is varied for tuning purposes by movement of the tuning plate 22 between the flange extensions 20.

If extreme miniaturization is desired or imperative, say for spot frequency operation as in key communications, it is quite easy to change the present model so that it will operate at a much lower frequency. The aperture will eventually have to be sealed and a window made of a suitable dielectric material whose thickness extends the full depth of the posts constructed. Another change is to reduce the width of the posts over the length between the side of the cavity and the cylindrical capacitor sleeve.

The power equalized operation of the tunable cavity antenna as a constant impedance transmitter or receiver of circularly polarized waves is by no means the only application for which it is well adapted. Replacing the damping resistor of the power equalizer by the proper circularly polarized ones.

kind of detector or receiver would permit the circularly polarized operation of a radar set. The antenna would transmit right circularly polarized waves and receive left Cross-talk to the receiver would be well down because of the excellent match of the radiators to the branch cables, and the usual highly refined TR box might be replaced by a much simpler one having a smaller insertion loss. It would, with tuning, be useable over a wide range.

It is feasible to offset the tuning of one element by a small amount so that the two elements would track very nearly at a fixed frequency difference over a wide frequency range. Perhaps the wide range of tracking would not be necessary, but a staggered-tuned arrangement could be used in telemetering applications or in any two wavelength responder system.

The elements of a cavity antenna might be used separately as individual fixed-tuned antennas performing entirely unrelated services, possibly at different frequencies and impedance levels. For instance, two transmitters or two receivers could be operated simultaneously through the same aperture independently of their respective frequencies.

If high power uses are contemplated, it may be necessary to take precautions against corona or actual flash-over at the probe tip or the gap. These are regions of high potential. The voltages may be reduced by reducing the value of the input resistance which may be accomplished by adjusting the capacitance of the probe.

The rigidity of the structure is of great importance. A one-piece casting might profitably comprise the four posts and the cavity shell exclusive of the back wall assembly, which could be bolted on. The dimensions or, rather, the shape of the inner conductor and probe are not especially critical. The required series capacitance may be obtained with any of a variety of shapes. The rigidity of the capacitor mounting is absolutely necessary, and a metal support is almost mandatory. A wobbly piston would be intolerable, and the alignment must be of the talk. The thickness, width, and the taper of the posts, and the depth of the cavity are not critical parameters, but they all affect the frequency of operation.

The invention is not intended to be limited to the examples of embodiments shown and described, but may on the contrary, be capable of many modifications without departing from the spirit of the invention.

We claim:

1. A miniature tunable cavity antenna comprising: a square metallic housing having continuous back and side walls and anopen front side, said front side of said housing being the aperture of said antenna; a pair of posts positioned in said aperture in facing relationship,

spaced apart and attached to the opposite sides of said housing, each of said postsis formed with a passage through its center; a coaxial line extending through the said passage in one of said posts; a probe excitor extendingfrom said coaxial line across the space between said posts into the passage of the other of said posts; a movable type capacitor element inside said housing for tuning said cavity antenna; and means for adjusting the position of the said capacitor.

2. A miniature tunable cavity antenna comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passages being at a different level for each pair of posts; a coaxial line extending through the said passage in one of said first pair of posts; a coaxial line extending through the passage in one of said highest precision to achieve good tracking and low cross second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; a movable piston type capacitor inside said housing for tuning said cavity antenna; and means for adjusting the position of the said capacitor.

3. A' miniature tunable cavity antenna comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of said antenna; means for exciting said aperture positioned in said aperture including orthogonally crossed radiators rigidly attached to said side walls of said housing forming a symmetrical-cross opening between said radiators, said crossed radiators being geometrically identical and entirely symmetrical in their arrangement and first and second cable means passing through the central portion of said radiators and crossing the said opening at right angles to each other for exciting said crossed radiators; a piston type capacitor element inside said housing; and means for adjusting the position of said capacitor for tuning said antenna.

4. A miniature tunable cavity antenna comprising: a metallic housing having continuous back and side walls and an open front side, said front side of said housing being between A; and wavelength square and is the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passage being at a different level for each pair of posts; a coaxial line extending through the said passage in one of said first pair of posts; a coaxial line extending through the passage in one of said second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; hollow cylindrical shielding means extending from the tips of said posts for shielding each of the said probe excitors to a small gap at the center of the aperture; a movable piston type capacitor inside said housing for tuning said cavity antenna; and means for adjusting the position of the said capacitor.

5. A miniature tunable cavity antenna comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passages being at a different level for each pair of posts; a coaxial line extending through the said passage in one of said first pair of posts; a coaxial line extending through the passage in one of said second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; flange cylindrical extensions attached to the bottom of each of said posts and to the said sides of the housing forming a circular opening; and a movable piston type capacitor inside said housing for tuning said cavity antenna; said capacitor including a circular tuning plate having a discontinuous cylindrical extension below said plate, and means for adjusting the position of the said capacitor within the said opening defined by the said flange extensions.

6. A miniature tunable cavity antenna for circularly polarized radiation comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of 7 said antenna; means for exciting said aperture positioned in said aperture including orthogonally crossed radiators rigidly attached to said side walls of said housing forming a symmetrical-cross opening between said radiators, said crossed radiators being geometrically identical and entirely symmetrical in their arrangement and cable means differing in electrical length by nominally 11'/ 2 radians from a common junction for exciting said crossed radiators; a piston type capacitor element inside said housing; and means for adjusting the position of said capacitor for tuning said antenna.

7. A miniature tunable cavity antenna for circularly polarized radiation comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of said antenna; means for exciting said aperture positioned in said aperture including orthogonally crossed radiators rigidly attached to said side walls of said housing forming a symmetrical-cross opening between said radiators, said crossed radiators being geometrically identical and entirely symmetrical in their arrangement, a power equalizer and first and second cable means passing through the central portion of said radiators and crossing the said opening at right angles to each other, the output of said power equalizer differing in electrical length by nominally 1r/ 2 radians for exciting said crossed radiators; a piston type capacitor element inside said housing; and means for adjusting the position of said capacitor for tuning said antenna.

8. A miniature tunable cavity antenna for circularly polarized radiation comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passages being at a different level for each pair of posts; a first coaxial line extending through the pasasge in one of said first pair of posts; a second coaxial line extending through the passage in one of said second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; a power equalizer; branch cables electrically connecting the output of said power equalizer with said first and second coaxial lines; said branch cables differing in electrical length by nominally w/Z radians; a movable piston type capacitor inside said housing for tuning said cavity antenna; and means for adjusting the position of the said capacitor.

9. A miniature tunable cavity antenna for circularly polarized radiation comprising: a square metallic housing having continuous back and side walls and an open front side, said front side of said housing being the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passages being at a different level for each pair of posts; a first coaxial line extending through the passage in one of said first pair of posts; a second coaxial line extending through the passage in one of said second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; a hollow cylindrical shielding means extending from the tips of said posts for shielding each of the said probe excitors to a small gap at the center of the aperture; a power equalizer; branch cables electrically connecting the output of said power equalizer with said first and second coaxial lines; said branch cables differing in electrical length by nominally 11'/2 radians; a movable piston type capacitor inside said housing for tuning said cavity antenna; and means for adjusting the position of the said capacitor.

10. A miniature tunable cavity antenna for circularly polarized radiation comprising: a square metallic housing having continuous backand side walls and an open front side, said front side of said housing being the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passages being at a different level for each pair of posts; a first coaxial line extending through the passage in one of said first pair of posts; a second coaxial line extending through the passage in one of said second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; a power equalizer; branch cables electrically connecting the output of said power equalizer with said first and second coaxial lines; said branch cables differing in electrical length by nominally 1r/2 radians; flange cylindrical extensions attached to the bottom of each of said posts and to the sides of the housing forming a discontinuous circular opening; and a movable piston type capacitor inside said housing for tuning said cavity antenna; said capacitor including a circular tuning plate having a continuous cylindrical extension below said plate, and means for adjusting the position of the said capacitor within the said opening defined by said flange extensions.

11. A miniature tunable cavity antenna for circularly polarized radiation comprising: a metallic housing having continuous back and side walls and an open front side, said front side of said housing being between Va and wavelength square and is the aperture of said antenna; first and second pairs of posts positioned in said aperture at right angles to each other and attached to the housing forming a symmetrical-cross opening between said pairs of posts; each of said posts is formed with a passage through its central portion; said passages being at a different level for each pair of posts; a first coaxial line extending through the passage in one of said first pair of posts; a second coaxial line extending through the passage in one of said second pair of posts; a probe excitor extending from each of said coaxial lines across the space between said posts into the passage of the other of said posts; a hollow cylindrical shielding means extending from the tips of said posts for shielding each of the said probe excitors to a small gap at the center of the aperture; a power equalizer; branch cables electrically connecting the output of said power equalizer with said first and second coaxial lines; said branch cables differing in electrical length by nominally 1r/2 radians; flange cylindrical extensions attached to the bottom of each of said posts and to the sides of the housing forming a discontinuous circular opening; and a movable piston type capacitor inside said housing for tuning said cavity antenna; said capacitor including a circular tuning plate having a continuous cylindrical extension below said plate, and means for adjusting the position of the said capacitor within the said opening defined by said flange extensions.

References Cited in the file of this patent UNITED STATES PATENTS 2,573,460 Lindenblad Oct. 30, 1951 tom- -4. 

